The invention relates to a process for cooling metal during casting as it emerges from a mold and this by applying coolant directly onto the surface of the metal, as a result of which at least during the start-up phase a gas is released from the coolant.
During continuous direct chill casting of metal the strand or ingot of metal is jetted with a coolant directly on the surface as the ingot leaves the mold i.e. heat is extracted from the metal immediately below the mold. During the start-up phase of casting the coolant first strikes only the dummy base. The resultant indirect extraction of heat produces a moderate rate of solidification of the liquid metal and flat shape of that end of the ingot. As the dummy base is lowered further, the coolant strikes the surface of the ingot directly, which causes a sudden increase in the rate of heat extraction from the ingot. The thermal stresses resulting from this thermal shock exceed the yield strength of the ingot and lead to a permanent deformation in the form of a convex curvature at the foot of the ingot; on exceeding the tensile strength of the material at that temperature they lead to tears or cracks in the ingot. In order to obtain an ingot with a flat starting end therefore, the ingot must not be cooled too intensively at the start-up of casting.
A process is known in which, at least at the start of casting, the coolant contains a gas introduced into it under pressure. As the coolant strikes the surface of the ingot, the gas thus dissolved in the coolant forms a thermally insulating layer which reduces the cooling intensity.
The disadvantages of this process are the expensive mixing and control facilities needed to dissolve the gas in the coolant and the fact that, because of the generally low solubility of gases in water which is principally used as coolant here, the process is practically limited to the use of carbon dioxide.